1. Field of the Invention
This invention relates to improvement of an electrostatic reproducing apparatus provided with a corona charge generator for detaching a transfer paper, having a lamp for exposing a photosensitive member before transfer and after development.
2. Description of the Prior Art
A conventional type of electrostatic reproducing apparatus has a pretransfer exposure lamp which irradiates the surface of a photosensitive member subjected to a toner development under a constant condition irrespective of change in a document and a transfer paper, and then a corona charge generator for detaching the transfer paper, i.e. a separating electrode, generates a charge onto the back of the transfer paper.
Generally, an electrostatic reproducing apparatus in which a transfer paper is detached by means of a separating electrode has no separating means coming in direct contact with the photosensitive member and hence is superior in having no possibility of damaging the photosensitive member or partly cutting a toner image as compared with a transfer type electrostatic reproducing apparatus using a separating claw and a separating belt. However, in a conventional electrostatic reproducing apparatus using a separating electrode, there may be a case where a transfer paper is not detached stably or transfer rate of the toner image changes from one type of document to another.
In case, for example, an area ratio of the photosensitive member is large at a portion where the surface potential is high, as in the case of a photo document, and an area ratio of the photosensitive member is small at a portion where the surface potential is high as in the case of a character document, a constant irradiation condition of the pretransfer exposure lamp and a constant charge generating condition of the separating electrode cannot ensure a detachment in the same way and can cause a difference in the transfer efficiency of the toner image.
Table 1 indicates the results obtained in examining a relation between a difference in the photosensitive member surface potential before development, which may arise in accordance as the document varies, and detachability of the transfer paper with changes in discharge current of the separating electrode, using the Se-Te system for the photosensitive member. The discharge current of the transfer electrode was 30 .mu.A (DC) in the test.
TABLE 1 ______________________________________ Photosensitive member surface potential and detachability Photosensitive member surface Separating electrode discharge current (.mu.A) potential (V) 30 60 90 120 150 180 ______________________________________ 0 X X X O O O 120 X X O O O O 600 O X X X X X ______________________________________ (Note) The symbol "O" in the above table indicates that the transfer paper transferred to an A4sized sheet 50 g/m.sup.2 in basis weight has been detached perfectly; the symbol "X" indicates that the transfer paper has not been detached perfectly.
The result given in Table 1 indicates that the transfer paper can be detached perfectly at all times if the discharge current of the separating electrode is changed to cope with any big change in the photosensitive member surface potential due to a change of the document. However, with reference to a detachment of the transfer paper, the discharge current of the separating electrode should relate largely to the surface potential of the photosensitive member after development rather than before toner development. Therefore, results given in FIG. 1 and FIG. 2 are obtained through examining the effect of the discharge current of the separating electrode.
In FIG. 1 and FIG. 2, V.sub.1 denotes the surface potential of a transfer paper 1, the same as that in Table 1, immediately after passing a separating electrode 2, and V.sub.2 denotes the surface potential of a photosensitive member 3 appearing on the lower side thereof, respectively as shown in FIG. 3. The surface potential V.sub.2 can be regarded as coming near to the surface potential of the photosensitive member after development. Then, FIG. 1 indicates a result when the photosensitive member surface potential in Table 1 is 600 V; FIG. 2 indicates a result when the photosensitive member surface potential is 120 V.
From comparing measured results of FIG. 1 and FIG. 2 with that of Table 1, it is understood that the transfer paper 1 is ready for perfect detachment when the surface potential V.sub.1 of the transfer paper 1 after passing the separating electrode 2 becomes almost equal to the surface potential V.sub.2 of the photosensitive member, appearing on the lower side thereof, and a perfect detachment will not be secured under the state wherein a relative potential difference is present between the two surface potentials V.sub.1, V.sub.2.
What is conceivable from the above is that the reason why the transfer paper 1 is drawn toward the photosensitive member 3 in FIG. 3 is that an electric field due to a charge on the photosensitive member 3 and an induced charge on the photo sensitive member substrate 4 works on a charge on the transfer paper 1, and the charge on the transfer paper 1 prevents detachment of the transfer paper 1 and also causes a relative potential difference between the surface potential V.sub.1 of the transfer paper 1 and the surface potential V.sub.2 of the photosensitive member 3 appearing on the lower side thereof, and when the charge is eliminated by the separating electrode 2, the transfer paper 1 is brought to a state like a conductive material, and an electrostatic adsorption of the transfer paper 1 is released for detachment. Be that as it may, FIG. 1 and FIG. 2 illustrate that for detachability of the transfer paper the discharge current required at the separating electrode is more dependent upon the photosensitive member surface potential after development than upon the photosensitive member surface potential before development. It is therefore preferable that the photosensitive member surface potential after development be grasped securely so as to detach the transfer paper perfectly at all times by controlling the discharge current of the separating electrode. Then, it has been found that there is a correlation between the photosensitive member surface potential and a current flowing in a development electrode at the time of development.
It was therefore conceived that density of the toner picture would be utilized. The toner picture density can be measured stably without contact by combining a light emitting element and a light receiving element.
FIG. 4A is a graph obtained through examining a relation between the photosensitive member surface potential before toner development and the toner picture density after development. For the toner picture density, an infrared LED having a peak at 9,500 .ANG. works as a light emitting element, a phototransistor works as a light receiving element, the light receiving element detects the strength of reflected light from the light emitting element on the photosensitive member surface before and after the photosensitive member is subjected to toner development, and the toner picture density is indicated by the output voltage from a density detecting circuit, corresponding to the difference between the two detection outputs of the light receiving element. As will be apparent from FIG. 4A, the toner picture density increases as the photosensitive member surface potential rises, but its rate of rise decreases suddenly when the photosensitive member surface potential exceeds 600 V. However, as will be understood from Table 1, the discharge current of the separating electrode will have to be changed substantially in the range of photosensitive member surface potential up to 600 V. Moreover, toner picture gives information about the photosensitive member surface after development, therefore it can be used for full control of the discharge current of the separating electrode.
The above represents the case where a pretransfer exposure is not carried out, however, the pretransfer exposure after development may deteriorate the surface potential of the photosensitive member. Therefore, from the results given in FIG. 1 and FIG. 2, detachability of the transfer paper will be changed naturally according to the pretransfer exposure.
Table 2 shows how the result of Table 1 will change according to the pretransfer exposure, indicating a detachability of the transfer paper when the photosensitive member surface is irradiated at 30 lux sec. with a cold cathode fluorescent tube having a peak at about 400 nm after toner development and before transfer. Conditions of the transfer paper and the photosensitive member are the same as for Table 1.
TABLE 2 ______________________________________ Detachability at pretransfer exposure Photosensitive member surface Separating electrode discharge current (.mu.A) potential (V) 80 100 120 140 160 180 ______________________________________ 0 X X X O O O 120 X X X O O O 600 X X O O O O ______________________________________
As will be apparent from the result given in Table 2, the transfer paper can be detached perfectly in the case of a photo document by carrying out the pretransfer exposure at 30 lux sec. under the same condition as a character document. However, since the pretransfer exposure may involve a fatigue on the photosensitive member, the quantity of light must be adjusted as little as possible.
FIG. 5 shows circumstances of fatigue of the surface potential of two photosensitive members which are electrified under the same conditions before exposure of an original image, one being not subjected to the pretransfer exposure and the other subjected to the pretransfer exposure under the same conditions as Table 2, and showing the change for repetitive transfer; it can be understood that the photosensitive member is considerably fatigued even by the pretransfer exposure at 30 lux sec. In FIG. 5, a photosensitive member of the Se-Te system is also used.
Table 3 and Table 4 show results obtained through examining the relation between the quantity of light of pretransfer exposure and effect; Table 3 shows the relation between rate of pretransfer exposure and detachability of the transfer paper, which is obtained through using various thicknesses of A4-sized transfer papers, and Table 4 shows the relation between rate of pretransfer exposure and transfer efficiency, i.e. the ratio of transfer toner quantity to development toner quantity. In both Table 3 and Table 4, the same conditions as for Table 2 are used for the photosensitive member and the pretransfer exposure lamp, a discharge current of the separating electrode is specified at 130 .mu.A, and a discharge current of the transfer electrode 5 shown in FIG. 3 is specified at 30 .mu.A. The transfer paper of Table 4 is 65 g/m.sup.2 in basis weight.
TABLE 3 ______________________________________ Rate of pretransfer exposure and detachability Pretransfer exposure Transfer paper (g/m.sup.2) (lux sec.) 50 65 127 ______________________________________ 0 0 35 100 13 0 100 100 24 20 100 100 30 100 100 100 ______________________________________
TABLE 4 ______________________________________ Rate of pretransfer exposure and transfer efficiency Pretransfer exposure Transfer efficiency (lux sec.) (%) ______________________________________ 0 70 15 80 33 88 ______________________________________
As will be apparent from Table 3, even a little quantity of pretransfer exposure is effective to improve the detachability according to the thickness of transfer paper, and further is influential, from Table 4, in improvement of transfer efficiency.
It was then conceived to utilize information toner developer current to get information about the photosensitive member surface potential easily, and to get information more directly related to the situation after development.
FIG. 4B indicates the relation between photosensitive member surface potential before development and developing current flowing in a development bias circuit when the photosensitive member surface at the potential is developed at a toner developer. As is self-explanatory in FIG. 4B, the developing current increases almost in proportion to the photosensitive member surface potential when the photosensitive member surface potential runs at 200 V or over, and then the discharge current of the separating electrode must be changed for stable detachment of the transfer paper, as indicated in Table 1, when the photosensitive member surface potential exceeds 200 V, therefore information on the developing current can be utilized for full control of the separating electrode for stable detachment of the transfer paper.